Bilateral sternalis with unusual left-sided presentation: a clinical perspective

An unusual variation creates interest among anatomists, but is a cause of concern among clinicians when it mimics a pathology. The sternalis muscle is one such variant of the anterior chest wall located subcutaneously over the pectoralis major, ranging from a few short fibers to a well-formed muscle. We observed a bilateral case, which was accompanied by an atypical presentation on the left side where a huge, bulky sternalis muscle was associated with the absence of the sternal fibers of the pectoralis major. The fibers arose as a lateral strip from the upper two-thirds of the body of the sternum and costal cartilages 2 through 6 with the intervening fascia and aponeurosis of the external oblique. The right sternalis was strap-like and was placed vertically over the sternal fibers of the pectoralis major, arising from the underlying fascia and aponeurosis of the external oblique. The sternalis muscles, on each side, converged into an aponeurosis over the manubrium that was continuous with the sternal heads of the right and left sternocleidomastoid muscle, respectively. This rare anomaly has puzzled radiologists and surgeons in confirming diagnosis, missing it all together or mistaking it for a tumor on mammography or CT scan. These findings prompted us to review its topography, development, and application in relation to the anterior chest wall.